This invention relates to an extruded, open-cell alkenyl aromatic polymer foam and a process for making.
Alkenyl aromatic polymer foams have been employed extensively in a variety of insulating applications. The most common types employed are closed-cell extruded foams and expanded bead foams. Extruded foams are made by forming a gel of a polymer melt and a blowing agent under heat and pressure, and extruding the gel through a die into a region of lower pressure. The lower pressure causes the gel to expand, cool, and solidify to form the foam. Bead foams are made by charging a mold with solid, expandable polymer beads containing an imbibed blowing agent or pre-expanded beads, and exposing the beads to heated air or steam to effect their expansion, coalescence, and adherence to form a unitary foam mass or article.
Heat distortion temperature of an alkenyl aromatic foam is very important when the foam is being used in a relatively high temperature application close to the service temperature limit (about 165.degree. F. for a conventional polystyrene foam) of the foam. If the heat distortion temperature of a foam is too low, it may be subject to disfigurement and/or rupture.
Extruded, closed-cell foams are more vapor-resistant, more water-resistant, and mechanically stronger than expanded bead foams. Extruded, closed-cell foams have these advantages because they are made in a solid, cellular form. Expanded bead foams, in contrast, are formed in a coalesced mass of discrete, expanded foam beads.
Expanded bead foams typically better maintain their shape in a high temperature environment than extruded, closed-cell foams because they typically have better bowing characteristics. Their bowing characteristics are better because the coalesced expanded beads allow for greater mechanical relaxation compared to the solid, cellular form of extruded, closed-cell foams.
A common high temperature application for alkenyl aromatic foams is in roofing. In roofing, the foam is typically employed below a roofing membrane, which is dark and rubber-like, and may reach service temperature limits when underneath a membrane exposed to direct sunlight in the summer months. If the foam becomes distorted, the membrane and the foam may separate to form void pockets, which leaves the membrane with less mechanical support on its undersurface. The lack of undersurface support renders the membrane more subject to rupture, which results in water leaking in the roof.
It would be desirable to have an alkenyl aromatic foam which had a vapor resistance, water resistance, and mechanical strength similar to that of extruded, closed-cell foams and bowing characteristics similar to that of expanded bead foams.